1. Field of the Invention
The invention relates generally to RF communication circuits, and, more specifically, to modulators.
2. Description of Related Art
A modulator is a type of mixer that combines a radio-frequency (RF) carrier with a modulating signal to develop a modulated output signal. One notable performance parameter for a modulator is linearity, which is defined as the degree to which the amplitude of the modulated output signal is proportional to the amplitude of the modulating signal. In operational environments where frequency modulation is employed, including AMPS (advanced mobile telephone service), modulator linearity is generally not a critical parameter. However, a number of recently-developed wireless communication protocols utilize modulation schemes that are much more complex than basic frequency modulation, placing special demands on modulator performance. Existing modulators may provide insufficient linearity to meet these demands. More specifically, in the context of modulation schemes used to implement CDMA (code division multiple access), TDMA (time division multiple access), GSM (global system for mobile), and other types of wireless communication protocols, a modulator must be capable of transmitting data at a relatively high speed while, at the same time, providing reasonably error-free performance. Modulator nonlinearities will introduce errors into the modulated data, degrading the quality and reliability of an RF communications link.
Various techniques have been developed to enhance amplifier linearity, but these techniques are not readily adapted for use with modulators. For example, consider the use of negative feedback. In an amplifier, the input signal is typically at the same frequency as the desired output signal, and, moreover, the amplifier does not substantially change the modulation of the signal from input to output. A suitable amplifier feedback signal can be developed by attenuating the output signal and, if necessary, inverting its phase. By contrast, in a modulator, the desired output signal is generally in a different frequency band than the modulating signal. Moreover, the output is modulated, whereas the input receives an unmodulated carrier. It is often difficult or impossible to utilize the modulator output in a manner so as to provide a useful feedback signal.
The manner in which the linearity of a modulator may be enhanced depends, to some extent, on the type of modulator under consideration. There are two basic types of modulators: balanced and unbalanced. Unbalanced modulators advantageously exploit the nonlinearity of an active semiconductor device to produce a mixing effect. The output signal includes all input signals and a plurality of mixing products. Although generally considered to be an undesired property, the existence of all input signals at the mixer output is helpful in that it allows one to readily construct a suitable feedback signal. But due to the lack of unmodulated carrier suppression, unbalanced modulators are not well-suited for applications other than simple frequency conversion.
Balanced modulators are mixers which combine a first signal with a second signal in a manner such that the mixer output is substantially free of the first signal. Double-balanced modulators are a type of balanced modulator which provide an output which is substantially free of the first and second signals. Unfortunately, it is difficult to develop a suitable feedback signal to improve the linearity of a balanced modulator. Due to the fact that the modulator output is substantially free of one or more of the unmixed input signals, signal components which could be used to develop a feedback signal are lacking.
One type of balanced mixer well-known to those skilled in the art is a Gilbert Cell. US Pat. No. 5,095,290 (hereinafter, the ""290 patent) discloses a modulator that uses negative feedback to improve the linearity of a Gilbert Cell. As with other types of balanced mixers, a Gilbert Cell lacks an output signal which is suitable for developing an appropriate feedback signal. The ""290 patent overcomes this problem by using a first and a second Gilbert Cell. A first half of each Gilbert Cell is used to provide the modulated output signal, and a second half of each Gilbert Cell is used to reconstruct the modulating signal from a temporally sliced signal that is generated by each Gilbert Cell. The first Gilbert Cell provides a temporally sliced waveform that is complimentary to the temporally sliced waveform provided by the second Gilbert Cell. When these two temporally sliced waveforms are combined, they produce a usable modulator feedback signal.
The modulator described in the ""290 patent presents a significant shortcoming. Although the temporally-sliced waveforms should theoretically fit together in a complementary manner, this requires a very high degree of matching between the active devices utilized in the Gilbert Cells. Accurate matching of device gain must be performed for each transistor-transistor pair within a Gilbert Cell. Additionally, the gain of each portion of the first Gilbert Cell must be closely matched to the gain of the corresponding portion of the second Gilbert Cell. If any transistor mismatches exist, the two temporally sliced waveforms will fit together poorly, if at all. Due to the fact that four pairs of transistors are used as differential amplifiers for the carrier signal, two pairs of transistors are used for the modulating signal, and two pairs of transistors are employed for the feedback signal, a significant amount of transistor matching must be performed.
Transistor matching procedures introduce added cost, complexity, and time delays into the semiconductor fabrication process. Moreover, the modulating signal is delayed in the modulator, resulting in a feedback signal that is out of phase with the modulated signal. It is not possible to reduce or eliminate all transistor-to-transistor mismatches. The remaining mismatches cause carrier imbalances whereby the carrier signal is not substantially cancelled out at the modulator output. In addition, the use of so many transistors and their corresponding interconnections provide numerous parasitic capacitance pathways that significantly limit high-frequency operation. What is needed is a modulator that provides improved linearity and that ameliorates the above-noted problems.
The linearity of a modulator is enhanced by filtering a set of frequency components from the modulator output to develop a modulator feedback signal. In a preferred embodiment of the invention, the output of a modulator is separated into a first frequency spectrum and a second frequency spectrum. The first frequency spectrum contains the modulated output signal, and the second frequency spectrum contains the modulator feedback signal which is combined with the modulating signal to enhance the linearity of the modulator.